An aromatic liquid-crystalline polyester is broadly used for electronic parts, such as connector, or electronic device due to the low moisture uptake, excellent heat resistance and thin-wall moldability thereof. Moreover, since aromatic liquid-crystalline polyesters have small dielectric loss in a high frequency region, it is utilized for multilayered printed circuit boards for high frequency.
Polyesters having a repeating unit derived from para-hydroxybenzoic acid as a main component have been known as conventional aromatic liquid-crystalline polyesters (EP 1319678 A1). Although aromatic liquid-crystalline polyesters having a repeating unit derived from para-hydroxybenzoic acid as a main component have a small dielectric loss in gigahertz bands, it does not have sufficient dielectric loss in frequency in megahertz bands. Further, conventional aromatic liquid-crystalline polyesters have large volume expansion.
On the other hand, thinner and lighter wiring boards are being required for achieving of improvement in performance, down-sizing, thinner and lighter wiring boards in electronic device. As methods for obtaining thinner and lighter wiring boards, a method of forming a dielectric film on a board by a vapor-depositing method or a sputtering method, and a method (US 2002/049270 A) of coating dielectric substance paste on a substrate to form a dielectric film, are known.
However, a method according to a vapor depositing or a sputtering require high temperature of not less than 500° C. for film forming, and cause degradation of electrode materials by high temperatures.
A method using dielectric substance pastes does not require the above-mentioned high temperature, it does not provide sufficient dielectric loss of a dielectric film.
Objects of the present invention are to provide an aromatic liquid-crystalline polyester having a small dielectric loss in a wide frequency region, and to provide a dielectric substance paste to give dielectric films having small dielectric loss.